At present time, bar, sheet, strip and other products are obtained from molten steel in several ways. The conventional old-fashioned way is to cast the molten steel into an ingot, roll the ingot down into a slab, bloom or billet and then into bars, sheet, strip or the like as desired. An ingot weighs many tons and is of oblong or square shape, the width being from about 20 inches up to 32 inches and the height around 6 feet or more. A "slab" is a relatively flat, elongated rectangle with a width of from 24 to 80 or more inches, a length of from 10 to 30 feet and a thickness of from 2 to 9 inches. A "bloom" is mostly square in the range of 6 .times. 6 inches up to 12 .times. 12 inches and at least 10 feet long. A "billet" is mostly square in the range of 2 .times. 2 inches up to 15 .times. 5 inches and at least 10 feet long.
In conventional steel making practice, molten steel is teemed or poured from a ladle into a cast iron ingot mold. The life of an ingot mold varies from 10 to 50 casts depending on the size of the ingot, its shape and temperature of the metal. The major cause of mold rejection is metal thermal fatigue caused by high temperature gradiants between the inside and outside of the mold. With repeated casts, these thermal gradiants cause high tensile stresses to develop in the ingot mold when it cools and they increase to the extent that cracks develop on the inside face. The cracks increase in size and permit penetration of molten metal so that it becomes difficult to strip the ingot from the mold. Ingots in this condition are referred to in the industry as "stickers." When a sticker develops, generally the mold is broken loose and then scrapped. Another reason for mold rejection is erosion of the mold caused by the molten metal stream impinging on the side of the mold as it is poured from the ladle.
Two factors have an important bearing on the quality of steel castings produced according to conventional practices. First, the ingot mold heats up considerably and expands. Secondly, the molten steel in the mold as it starts to solidify shrinks away from the wall of the expanded ingot mold. These two factors combine to cause an air gap to form between the outside of the casting and the hot face of the ingot. The development of the air gap reduces the heat transfer from the ingot to the mold and increases the time necessary for solidification.
After ingots are poured and stripped from the molds, they are placed in a soaking pit to equalize the temperature throughout the ingot prior to rolling. The ingot may stay in the soaking pit as long as 24 hours, all of which requires a large volume of soaking pit gas to maintain a uniform temperature. After soaking, the ingot is usually rolled on a mill to form slabs, blooms or billets and the power required to reduce the ingot to these forms is considerable.
It will be apparent from the above description that considerable time and expense is involved in the steps of teeming ingots, stripping the molds from the ingots, equalizing the temperature of the ingots in the soaking pit, and subsequent rolling of the ingots to form blooms, slabs and billets. Accordingly, attention has been devoted to developing techniques for casting refined molten steel directly into slabs in order to avoid the time-consuming and expensive steps described above.
A more modern technique for making slabs, blooms and billets is the continuous casting process whereby molten steel is poured into a tundish, from there into vertical molds and then withdrawn by rolls or other mechanism. Lengths are cut off to give slabs, blooms or billets. While this technique is deceptively simple in practice, in principle it presents many inherent difficulties and the capital investment is very large.
Another technique is the bottom pressure casting method as described in my U.S. Pat. No. 3,196,503. According to the bottom pressure casting method, a ladle filled with molten steel is placed in a pressure vessel which is sealed with a lid. A pouring tube extends through the lid down to approximately 2 inches from the bottom of the ladle. The top part of the pouring tube is mechanically engaged to the filling end of the slab casting mold. Air pressure within the vessel causes the molten steel to rise through the pouring tube and enter the mold which is located at a slight tilt so that the molten metal enters the lower end. Usually a riser is provided on the mold. The mold is then supported in the relatively flat position until the molten metal solidifies to form a cast slab. The shrinkage of the metal quickly causes a gap to occur between the top surface of the molten metal and the wall of the mold. Since the hot metal goes to the top, inclusions and dirt tend to float to the top and this results in a defective surface on the top of the slab. When these slabs are rolled, the top surface of the rolled product is unsatisfactory and it is often necessary to burn off the top portion.
The apparatus of the present invention reduces the difficulties described above and afford other features and advantages heretofore not obtainable.
It is among the objects of the invention to cast refined ferrous metal directly into slabs with lower production costs, at higher production rates in a manner that maximizes the yield from the molten metal being poured, minimizes losses, and produces slabs with improved surface quality.